1. Field of the Invention
This invention relates to stress-crack resistant ethyleneperhaloethylene copolymers, particularly copolymers of 40 to 60 mol percent of ethylene with correspondingly from 40 to 60 mol percent of chlorotrifluoroethylene, tetrafluoroethylene or mixtures thereof.
2. Description of the Prior Art
Copolymers of about 40 to about 60 mol percent of ethylene with correspondingly from about 60 to about 40 mol percent chlorotrifluoroethylene, tetrafluoroethylene or mixtures thereof are known in the art and are disclosed, for example, in U.S. Pat. No. 3,847,881 issued in 1974 to Mueller et al. While such copolymers have high melting points, generally in the order of about 200.degree. C, their use at temperatures above about 150.degree. C has been severely restricted due to their tendency to thermal stress-crack at such temperatures. At such temperatures, the copolymers exhibit cracks under tensile stresses appreciably below the limits of their shortterm strength, especially when powder coated onto metal substrates.
To overcome the thermal stress-cracking tendency of ethylene/chlorotrifluoroethylene and ethylene/tetrafluoroethylene copolymers, it has been suggested in U.S. Pat. No. 3,624,250 issued in 1971 to Carlson to incorporate in such copolymers small amounts of a copolymerizable vinyl monomer. It has also been suggested in the aforesaid Mueller et al. patent to incorporate into copolymers of ethylene and halogenated comonomers selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and mixtures thereof, a minor amount of 3,3,3-trifluoro-2-trifluoromethyl propene to improve the thermal stress-crack resistance of the copolymers. It would be desirable to provide a copolymer of ethylene and a halogenated comonomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and mixtures thereof having improved thermal stress-crack resistance without necessitating the addition of a termonomer which adds to the cost of the copolymer.